In a typical radio communications network, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a Radio Base Station (RBS), which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not co-located. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. The base stations communicate over the radio interface operating on radio frequencies with the user equipments within range of the base stations.
In some versions of the RAN, several base stations are typically connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks. A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity.
Specifications for the Evolved Packet System (EPS) have been completed within the 3GPP and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations nodes, e.g., eNodeBs in LTE, and the core network. As such, the RAN of an EPS has an essentially “flat” architecture comprising radio base station nodes without reporting to RNCs.
Improved support for heterogeneous network operations is part of the ongoing specification of 3GPP LTE Release-10, and further improvements are discussed in the context of new features for Release-11. In heterogeneous networks, a mixture of cells of differently sized and overlapping coverage areas are deployed. One example of such deployments is where pico cells are deployed within the coverage area of a macro cell. Other examples of low power nodes, also referred to as points, in heterogeneous networks are home base stations and relays. The aim of deploying low power nodes such as pico base stations within the macro coverage area is to improve system capacity by means of cell splitting gains as well as to provide users with wide area experience of very high speed data access throughout the network. Heterogeneous deployments are in particular effective to cover traffic hotspots, i.e. small geographical areas with high user densities served by e.g. pico cells, and they represent an alternative deployment to denser macro networks.
LTE is a Frequency Division Multiplexing technology wherein Orthogonal Frequency Division Multiplexing (OFDM) is used in a Downlink (DL) transmission from a radio base station to a user equipment. Single Carrier-Frequency Domain Multiple Access (SC-FDMA) is used in an uplink (UL) transmission from the user equipment to the radio base station. Services in LTE are supported in the packet switched domain. The SC-FDMA used in the UL is also referred to as Discrete Fourier Transform Spread (DFTS)-OFDM. Hence, LTE uses OFDM in the DL and DFTS-OFDM in the UL. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each Resource Element (RE) corresponds to one subcarrier during one OFDM symbol interval on a particular antenna port. An antenna port is defined such that the channel over which a symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed. There is one resource grid per antenna port. A symbol interval comprises a cyclic prefix (cp), which cp is a prefixing of a symbol with a repetition of the end of the symbol to act as a guard band between symbols and/or facilitate frequency domain processing. Frequencies f or subcarriers having a subcarrier spacing Δf are defined along an z-axis and symbols are defined along an x-axis.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame comprising ten equally-sized subframes, denoted #0-#9, each with a Tsubframe=1 ms of length in time as shown in FIG. 2. A subframe is divided into two slots, each of 0.5 ms time duration. Furthermore, the resource allocation in LTE is typically described in terms of Resource Blocks (RB), where a resource block corresponds to one slot of 0.5 ms in the time domain and 12 contiguous 15 kHz subcarriers in the frequency domain. Two in time consecutive resource blocks represent a resource block pair and corresponds to the time interval upon which scheduling operates.
Resource blocks are numbered in the frequency domain, starting with resource block 0 from one end of the system bandwidth.
Transmissions in LTE are dynamically scheduled in each subframe where the base station transmits downlink assignments/uplink grants to certain UEs via the physical downlink control information such as Physical Downlink Control Channel (PDCCH) and enhanced PDCCH (ePDCCH). The PDCCHs are transmitted in the first OFDM symbol(s) in each subframe and spans, more or less, the whole system bandwidth. A UE that has decoded a downlink assignment, carried by a PDCCH, knows which resource elements in the subframe that contain data aimed for the UE. Similarly, upon receiving an uplink grant, the UE knows which time/frequency resources it should transmit upon. In LTE downlink, data is carried by the physical downlink shared data link on Physical Downlink Shared Channel (PDSCH) and in the uplink the corresponding link is referred to as the physical uplink shared link Physical Uplink Shared Channel (PUSCH).
Definition of enhanced downlink control signaling, i.e. ePDCCH, is ongoing in 3GPP. However, it is likely that such control signaling will have similar functionalities as PDCCH, with the fundamental difference of requiring UE specific Demodulation Reference Signals (DMRS) instead of Cell specific Reference Symbols (CRS) for its demodulation. One advantage is that UE specific spatial processing may be exploited for ePDCCH.
Demodulation of sent data requires estimation of the radio channel which is done by using transmitted reference symbols (RS), i.e. symbols known by the receiver. In LTE, CRS are transmitted in all downlink subframes and in addition to assist downlink channel estimation they are also used for mobility measurements performed by the UEs. LTE also supports UE specific RS aimed only for assisting channel estimation for demodulation purposes. The control information for a given user equipment is transmitted using one or PDCCH. This control signaling is typically transmitted in a control region comprising the first n=1, 2, 3 or 4 OFDM symbols in each subframe, where n is the Control Format Indicator (CFI). The downlink subframe also comprises CRS, which are known to the receiver and used for coherent demodulation of e.g. the control information. A downlink system with 3 OFDM symbols allocated for control signaling, for example the PDCCH, is illustrated in FIG. 3 and the 3 OFDM symbols are denoted as control region. The resource elements used for control signaling are indicated with wave-formed lines, resource elements used for data are indicated as white REs, and resource elements used for reference symbols are indicated with diagonal lines. Frequencies f or subcarriers are defined along an z-axis and symbols are defined along an x-axis. FIG. 3 illustrates how the mapping of physical control/data channels and signals can be done on resource elements within a downlink subframe. In this example, the PDCCHs occupy the first out of three possible OFDM symbols, so in this particular case the mapping of data could start already at the second OFDM symbol. Since the CRS is common to all UEs in the cell, the transmission of CRS cannot be easily adapted to suit the needs of a particular UE. This is in contrast to UE specific RS which means that each UE has RS of its own placed in the data region of FIG. 3 as part of PDSCH.
The length of the control region, which can vary on subframe basis, is conveyed in the Physical Control Format Indicator Channel (PCFICH). The PCFICH is transmitted within control region, at locations known by terminals. After a terminal has decoded the PCFICH, it thus knows the size of the control region and in which OFDM symbol the data transmission starts.
Also transmitted in the control region is the Physical Hybrid Automatic Repeat Request (Hybrid-ARQ or HARQ) Indicator, which carries Acknowledgement/Non-Acknowledgement (ACK/NACK) responses to the UE to inform if the uplink data transmission in a previous subframe was successfully decoded by the base station or not.
As previously indicated, CRS are not the only reference symbols available in LTE. As of LTE Release-10, a new RS concept was introduced with separate UE specific RS for demodulation of PDSCH and RS for measuring the channel for the purpose of Channel State Information (CSI) feedback from the UE. The latter is referred to as CSI-RS. CSI-RS are not transmitted in every subframe and they are generally sparser in time and frequency than RS used for demodulation. CSI-RS transmissions may occur every 5th, 10th, 20th, 40th, or 80th subframe according to a Radio Resource Control (RRC) configured periodicity parameter and an RRC configured subframe offset.
A UE operating in connected mode can be requested by the base station to perform CSI reporting, e.g. reporting a suitable Rank Indicator (RI), one or more Precoding Matrix Indices (PMI) and a Channel Quality Indicator (CQI). Other types of CSI are also conceivable including explicit channel feedback and interference covariance feedback. The CSI feedback assists the base station in scheduling, including deciding the subframe and RBs for the transmission, which transmission scheme/precoder to use, as well as provides information on a proper user bit rate for the transmission, link adaptation. In LTE, both periodic and aperiodic CSI reporting is supported. In the case of periodic CSI reporting, the UE reports the CSI measurements on a configured periodical time basis on the physical uplink control signaling (PUCCH), whereas with aperiodic reporting the CSI feedback is transmitted on the PUSCH at pre-specified time instants after receiving the CSI grant from the base station. With aperiodic CSI reports, the base station can thus request CSI reflecting downlink radio conditions in a particular subframe.
A detailed illustration of which resource elements within a resource block pair that may potentially be occupied by the new UE specific RS and CSI-RS is provided in FIG. 4. The CSI-RS utilizes an orthogonal cover code of length two to overlay two antenna ports on two consecutive REs. As seen, many different CSI-RS pattern are available. For the case of 2 CSI-RS antenna ports we see that there are 20 different patterns within a subframe. The corresponding number of patterns is 10 and 5 for 4 and 8 CSI-RS antenna ports, respectively. For Time Division Duplexing (TDD), some additional CSI-RS patterns are available.
Subsequently, the term CSI-RS resource may be mentioned. In such a case, a resource corresponds to a particular pattern present in a particular subframe. Thus two different patterns in the same subframe or the same CSI-RS pattern but in different subframes in both cases constitute two separate CSI-RS resources.
The CSI-RS patterns may also correspond to so-called zero-power CSI-RS, also referred to as muted REs. Zero-power CSI-RS corresponds to a CSI-RS pattern whose REs are silent, i.e., there is no transmitted signal on those REs. Such silent patterns are configured with a resolution corresponding to the 4 antenna port CSI-RS patterns. Hence, the smallest unit to silence corresponds to four REs.
The purpose of zero-power CSI-RS is to raise the Signal to Interference plus Noise Ratio (SINR) for CSI-RS in a cell by configuring zero-power CSI-RS in interfering cells so that the REs otherwise causing the interference are silent. Thus, a CSI-RS pattern in a certain cell is matched with a corresponding zero-power CSI-RS pattern in interfering cells. Raising the SINR level for CSI-RS measurements is particularly important in applications such as Coordinated Multi Point (CoMP) or in heterogeneous deployments. In CoMP, the UE is likely to need to measure the channel from non-serving cells and interference from the much stronger serving cell would in that case be devastating. Zero-power CSI-RS is also needed in heterogeneous deployments where zero-power CSI-RS in the macro-layer is configured so that it coincides with CSI-RS transmissions in the pico-layer. This avoids strong interference from macro nodes when UEs measure the channel to a pico node.
The PDSCH is mapped around the REs occupied by CSI-RS and zero-power CSI-RS so it is important that both the network and the UE are assuming the same CSI-RS/zero power CSI-RS configuration or else the UE is unable to decode the PDSCH in subframes containing CSI-RS or their zero-power counterparts.
Control Signaling in LTE Rel.8 to Rel.10
Messages transmitted over the radio link to users can be broadly classified as control messages or data messages. Control messages are used to facilitate the proper operation of the system as well as proper operation of each UE within the system. Control messages could include commands to control functions such as the transmitted power from a UE, signaling of RBs within which the data is to be received by the UE or transmitted from the UE and so on.
Examples of control messages are the PDCCHs which for example carry scheduling information and power control messages, the Physical HARQ Indicator Channel (PHICH), which carries ACK/NACK in response to a previous uplink transmission and the Physical Broadcast Channel (PBCH) which carries system information. Also the primary and secondary synchronization signals (PSS/SSS) can be seen as control signals with fixed locations and periodicity in time and frequency so that UEs that initially access the network can find them and synchronize.
The PBCH is not scheduled by a PDCCH transmission but has a fixed location relative to the PSS and SSSs. Therefore can the UE receive the system information transmitted in broadcast channel (BCH) before it is able to read the PDCCH.
In LTE Rel-10, all control messages to UEs are demodulated using the common or cell-specific reference signals (CRS) hence they have a cell wide coverage to reach all UEs in the cell without having knowledge about their position. An exception is the PSS and SSS which are stand-alone and does not need reception of CRS before demodulation. The first one to four OFDM symbols, depending on the configuration, in a subframe are reserved to contain such control information, see FIG. 3 and FIG. 5. Control messages could be categorized into those types of messages that need to be sent only to one UE, UE-specific control, and those that need to be sent to all UEs or some subset of UEs numbering more than one, common control, within the cell being covered by the eNB
Control messages of PDCCH type are demodulated using CRS and transmitted in multiples of units called Control Channel Elements (CCE) where each CCE contains 36 REs. A PDCCH may have Aggregation Level (AL) of 1, 2, 4 or 8 CCEs to allow for link adaptation of the control message. Furthermore, each CCE is mapped to nine Resource Element Groups (REG) comprising four REs each. These REG are distributed over the whole system bandwidth to provide frequency diversity for a CCE. Hence, the PDCCH, which consists of up to 8 CCEs, spans the entire system bandwidth in the first one to four OFDM symbols, depending on the configuration. FIG. 5 discloses a mapping of 1 CCE belonging to a PDCCH to the control region which spans the whole system bandwidth.
Enhanced Control Signaling in Rel.11
FIG. 6 shows a Downlink subframe showing 10 RB pairs and configuration of three ePDCCH regions, marked black, of size 1 Physical Resource Block (PRB) pair each. The remaining RB pairs can be used for PDSCH transmissions. In LTE Rel.11 it has been agreed to introduce UE-specific transmission for control information in form of enhanced control channels by allowing the transmission of generic control messages to a UE using such transmissions be based on UE-specific reference signals and by placement in the data region, see FIG. 6. This is commonly known as the enhanced PDCCH (ePDCCH), enhanced PHICH (ePHICH) and so on. For the enhanced control channel in Rel.11 it has been agreed to use antenna port pε{107,108,109,110} for demodulation, see FIG. 7 for normal subframes and normal cyclic prefix. FIG. 7 shows an example of UE-specific reference symbols used for ePDCCH in LTE. R7 and R9 represent the DMRS corresponding to antenna port 107 and 109 respectively. In addition can antenna port 108 and 110 be obtained by applying an orthogonal cover as (1,−1) over adjacent pars of R7 and R9 respectively.
This enhancement means that precoding gains can be achieved also for the control channels. Another benefit is that different PRB pairs, or enhanced control regions, see FIG. 9 below, can be allocated to different cells or different transmission points within a cell, and thereby can inter-cell or inter-point interference coordination between control channels be achieved. This is especially useful for HetNet scenario as will be discussed in the next section.
Enhanced Control Signaling for HetNet and CoMP
The concept of a point is heavily used in conjunction with techniques for Coordinated Multipoint (CoMP). In this context, a point corresponds to a set of antennas covering essentially the same geographical area in a similar manner. Thus a point might correspond to one of the sectors at a site, but it may also correspond to a site having one or more antennas all intending to cover a similar geographical area. Often, different points represent different sites. Antennas correspond to different points when they are sufficiently geographically separated and/or having antenna diagrams pointing in sufficiently different directions. Techniques for CoMP entail introducing dependencies in the scheduling or transmission/reception among different points, in contrast to conventional cellular systems where a point from a scheduling point of view is operated more or less independently from the other points. DL CoMP operations may include, e.g., serving a certain UE from multiple points, either at different time instances or for a given subframe, on overlapping or not overlapping parts of the spectrum. Dynamic switching between transmission points serving a certain UE is often termed as Dynamic Point Selection (DPS). Simultaneously serving a UE from multiple points on overlapping resources is often termed as joint transmission (JT). The point selection may be based, e.g., on instantaneous conditions of the channels, interference or traffic. CoMP operations are intended to be performed, e.g., for data channels such as PDSCH and/or control channels such as ePDCCH.
The same enhanced control region, see FIG. 10, can be used in different transmission points within a cell or belong to different cells, that are not highly interfering each other. A typical case is the shared cell scenario, where a macro cell contains lower power pico nodes within its coverage area, having (or being associated to) the same synchronization signal/cell ID, see FIG. 8. In pico nodes which are geographically separated, as B and C in FIG. 8, the same enhanced control region, i.e. the same PRBs used for the ePDCCH can be re-used. In this manner the total control channel capacity in the shared cell will increase since a given PRB resource is re-used, potentially multiple times, in different parts of the cell. This ensures that area splitting gains are obtained. FIG. 8 shows a heterogeneous network scenario where the dashed line indicates the macro cell coverage area and A, B and C corresponds to the coverage of three pico nodes. In a shared cell scenario A, B, C and the macrocell have the same cell ID, e.g. the same synchronization signal, i.e. transmitted or being associated to the same synchronization signal.
An example is given in FIG. 9 where pico node B and C share the enhanced control region whereas A, due to the proximity to B, is of risk of interfering with each other and is therefore assigned an enhanced control region which is non-overlapping. Interference coordination between pico nodes A and B, or equivalently transmission point A and B, within a shared cell is thereby achieved. In some cases, a UE may need to receive part of the control channel signaling from the macro cell and the other part of the control signaling from the nearby Pico cell.
This area splitting and control channel frequency coordination is not possible with the PDCCH since the PDCCH spans the whole bandwidth. The PDCCH does not provide possibility to use UE specific precoding since it relies on the use of CRS for demodulation.
FIG. 10 shows an ePDCCH which, similar to the CCE in the PDCCH, is divided into multiple groups and mapped to one of the enhanced control regions. Note that in FIG. 10, the enhanced control region does not start at OFDM symbol zero, to accommodate simultaneous transmission of a PDCCH in the subframe. However, as was mentioned above, there may be carrier types in future LTE releases that do not have a PDCCH, in which case the enhanced control region could start from OFDM symbol zero within the subframe.
Distributed Transmission of Enhanced Control Signalling
Even if the enhanced control channel enables UE specific precoding and such localized transmission as illustrated in FIG. 10, it can in some cases be useful to be able to transmit an enhanced control channel in a broadcasted, wide area coverage fashion. This is useful if the eNB does not have reliable information to perform precoding towards a certain UE, then a wide area coverage transmission is more robust.
Another case is when the particular control message is intended to more than one UE, in this case UE specific precoding cannot be used. An example is the transmission of the common control information using PDCCH, i.e. in the common search space (CSS).
In any of these cases can a distributed transmission over enhanced control regions be used, see FIG. 11 for an example, where the 4 parts belonging to the same ePDCCH are distributed over the enhanced control regions. FIG. 11 shows a downlink subframe showing a CCE belonging to an ePDCCH is mapped to multiple of the enhanced control regions, to achieve distributed transmission and frequency diversity or subband precoding.
It has been agreed in the 3GPP ePDCCH development that both distributed and localized transmission of an ePDCCH should be supported corresponding to FIG. 11 and FIG. 10 respectively. When distributed transmission is used, then it is also beneficial if antenna diversity can be achieved to maximize the diversity order of an ePDCCH message. On the other hand, sometimes only wideband channel quality and wideband precoding information is available at the eNB for which it could be useful to perform a distributed transmission but with UE specific, wideband, precoding.
One fundamental property of DL CoMP is the possibility to transmit different signals and/or channels from different geographical locations or points. One of the principles guiding the design of the LTE system is transparency of the network to the UE. In other words, the UE is able to demodulate and decode its intended channels without specific knowledge of scheduling assignments for other UEs or network deployments.
For example, different Downlink Control Information (DCI) messages on ePDCCH may be transmitted from ports belonging to different transmission points. Even though there are several reasons for serving a UE with control signaling from different points, one application consists of distributing parts of the scheduling algorithm at different points, such that, e.g., DL transmissions are associated to a different point than UL transmissions. In such a case, it makes sense to schedule DL and UL transmissions with control signaling providing directly from the respective points. A further application consists of serving a UE with parallel data transmissions from different points, e.g., for increasing data rate or during handover between points. A further application consists of transmitting system control information from a “master” point and rely on data transmission from other points, typically associated to pico nodes.
In all the above applications it makes sense to have the possibility to serve the UE with control signaling on ePDCCH from different points in the same subframe. In any case, UEs are not aware of the geographical location from which each RS port is transmitted. DMRS or UE specific RS are employed for demodulation of data channels and possibly certain control channels, ePDCCH. UE specific RS relieves the UE from having to know many of the properties of the transmission and thus allows flexible transmission schemes to be used form the network side. This is referred to as transmission transparency, with respect to the UE. A problem is however that the estimation accuracy of UE specific RS may not be good enough in some situations.
Geographical separation of RS ports implies that instantaneous channel coefficients from each port towards the UE are in general different. Furthermore, even the statistical properties of the channels for different ports and RS types may be significantly different. Example of such statistical properties include the received power for each port, the delay spread, the Doppler spread, the received timing, i.e., the timing of the first significant channel tap, the number of significant channel taps, the frequency shift. In LTE, nothing can be assumed about the properties of the channel corresponding to an antenna port based on the properties of the channel of another antenna port. This is in fact key part of maintaining transmission transparency.
Based on the above observations, the UE needs to perform independent estimation for each RS port of interest for each RS. This results in occasionally inadequate channel estimation quality for certain RS ports, leading to undesirable link and performance degradation of the radio communications network.